President Barack Obama will travel to Boston on Thursday for a memorial.

'Someone knows'

FBI Special Agent Richard DesLauriers told a news conference that pieces of nylon had been recovered from the scene, along with fragments of ball bearings and nails that were "possibly contained in a pressure-cooker device".

He said they were being sent to the agency's laboratory in Quantico, Virginia, where experts would reconstruct the devices to determine their make-up and components.

"The investigation is in its infancy," he added. "There are no claims of responsibility and the range of suspects and motives remains wide open."

Mr DesLauriers urged people to report anyone they had seen acting suspiciously.

"Someone knows who did this," he said.

The Associated Press quoted a source close to the investigation as saying that the bombs consisted of explosives placed in 1.6-gallon (6-litre) pressure cookers, one with shards of metal and ball bearings, the other with nails.

The bombs were put into black bags and left on the ground, the source said.

It has been reported that a circuit board and battery pack - parts of a triggering mechanism - were recovered.

Pressure cookers are cheap and easy to buy. Details of a pressure cooker bomb were once published in an online al-Qaeda magazine. It was one of three devices used in the 2010 attempted Times Square attack.

The US Dept of Homeland Security has issued two previous warnings about the dangers of pressure cooker bombs. The cooker is a tightly-sealed pot, which can increase the force of the blast when packed with explosives.

The explosion can be triggered using a simple device like a digital watch or mobile phone.

The Boston explosions contained metal shards, nails and other types of shrapnel, designed to cause maximum injury. Many of those wounded in the Boston attack suffered lower leg injuries and shrapnel wounds.

He said the attack was "heinous and cowardly" but stressed it was not yet known whether an organisation - either domestic or foreign - or a "malevolent individual" was responsible, nor what the motive might have been.

"Everything else at this point is speculation," he said.

"It will take time... but we will find whoever harmed our citizens and we will bring them to justice."

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Krystle Campbell's mother said the attack made no sense

One of those who died has been named as eight-year-old Martin Richard, from the Dorchester area of Boston.

He was at the finishing line with his mother and sister, who were both seriously injured.

"They were looking in the crowd as the runners were coming to see if they could identify some of their friends when the bomb hit," Congressman Stephen Lynch, a friend of the Richard family, told Associated Press.

Flowers and tributes were being placed outside the family's home on Tuesday.

A second victim was named as Krystle Campbell, a 29-year-old restaurant manager.

Her mother, Patty, shaking with emotion, told reporters that "you couldn't ask for a better daughter".

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"Everybody who knew her loved her... she had a heart of gold. This doesn't make any sense."

Boston University said the third person who died was a graduate student.

The Chinese consulate in New York confirmed that she was a Chinese national. "At the request of her family, the victim's personal information will not be disclosed," said a statement from the consulate.

She has been identified, however, by a Hong Kong TV channel which said she was from Shenyang in north-eastern China.

The university said she was with two friends watching the race near the finish line. One of the friends, named as Zhou Danling by Chinese TV, was said to be in a stable condition in hospital.

The consulate said it was providing "all necessary assistance" to the two families.

The first explosion went off close to the finish line at about 14:50 local time (18:50 GMT) on Monday.

Seconds later, as rescuers were rushing to help the injured, another explosion went off nearby.

Police have asked the public to send in any videos or photographs they may have from the day.

The London Marathon - the next major international marathon - is to go ahead on Sunday, with police saying they have well-rehearsed security plans.

Organisers have said they will hold a 30-second silence at the start as a mark of respect.

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